1. Field of the Invention
This invention relates to a process for separating a hydrocarbon feed gas mixture comprising at least C.sub.1 -C.sub.2 constituents including ethylene to produce a separated ethylene product.
2. Description of the Prior Art
Ethylene is one of the most important and largest commercial volume petrochemicals in the world today. As a result significant and continuing effort has been expended in the design and development of new and improved ethylene production, recovery, separation and purification methods.
Ethylene is primarily produced by thermal pyrolysis of hydrocarbon fractions. Heretofore, ethane and propane were the prevailing raw materials for ethylene production. However, the presently dwindling supply of these light hydrocarbons is expected to shift the predominant feedstocks for ethylene production to the more available heavier hydrocarbon fractions such as naphtha.
In commercial practice, naphtha feed stocks are pyrolyzed to produce a hydrocarbon gas mixture containing ethylene. To recover the end ethylene product, it is then necessary to separate the ethylene from the remaining hydrocarbon constituents and purify same.
In an ethylene plant, the sequence in which the various separation (fractionation) steps are employed has a significant influence on the capital and operating costs and energy requirements of the ethylene production facility. Pressure levels must be selected for the operation of each fractionation column in the ethylene plant in relation to this sequence and to the operating constraints for permissible column bottom temperatures and column overhead refrigeration levels. In each fractionation operation, there are maximum temperature limits inherent in the process step to avoid polymerization and fouling, and minimum temperature limits to avoid hydrocarbon freezing or formation of hydrate species.
In practice, the prior art has employed both low pressure, e.g., 150-200 psia, and high pressure, e.g., 500-600 psia, ethylene production processes. These processes, however, as they have evolved to date, both possess inherent deficiencies in overall energy utilization. In many instances, low pressure ethylene production processes are attractive since they allow easily facilitated fractionation of the various pyrolysis gas constituents, since lower pressures in general provide higher relative volatilities. In turn, higher relative volatilities permit lower reflux ratios and correspondingly lower condenser heat duties to be employed for the various fractionation columns in the ethylene plant. Nonetheless, the low process temperatures associated with low pressure operation substantially increase the refrigeration load requirements for the ethylene plant and, attendantly, the compression requirements associated with the refrigeration system for the ethylene plant. High pressure operation, on the other hand, overcomes many of the inherent deficiencies associated with low temperature operation, and is generally preferred in practice. Nonetheless, at high operating pressures, the decrease in relative volatilities with the corresponding increas in reflux ratios, which are required to carry out the necessary sparation steps, substantially raise the condenser heat duties for the fractionation columns in the ethylene plant. Relative to the low pressure ethylene production process, the high pressure process has a substantially lower compression requirement associated with the ethylene plant refrigeration system. Despite such relative advantage, the refrigeration system compression requirement in the high pressure process is still large in magnitude and when added to the large compression requirements for the pyrolyzed feed gas mixture yields a high overall compression requirement for the process.
In connection with the foregoing, a significant operating cost in any ethylene production plant is associated with the compression system therefor. There are two functional requirements which the conventional ethylene plant compression system satisfies. First of all, the pyrolyzed hydrocarbon gas mixture must be pressurized to permit acid gas removal, to facilitate the recovery of the heavy hydrocarbon fractions the therefrom, and to minimize the overall refrigeration requirements in recovering the lighter hydrocarbon constituents, including ethylene. Secondly, the refrigeration system in the ethylene plant requires considerable compression energy. In the refrigeration system, the refrigerant fluids undergo a closed cycle of compression and expansion to supply the necessary cold for provision of the heat duty for the condensers of the light hydrocarbon fractionation columns.
In conventional high pressure ethylene plants, wherein the low temperature fractionation steps for separating the constituents of the ethylene-containing hydrocarbon feed gas mixture are conducted at pressure levels on the order of 300-600 psia, the refrigeration for the separation steps is supplied by propylene refrigerant at temperature levels down to about -40.degree. C. and by ethylene refrigerant at levels down to about -100.degree. C.
In low pressure ethylene plants, methane refrigeration has been employed in conjunction with propylene and ethylene refrigerants in cascaded refrigeration systems. Such cascaded systems permit temperatures as low as -130.degree. C. to be achieved and allow the operating pressures of the fractionation steps to be substantially reduced, for example, to 150-200 psia, by virture of the lower temperatures. Triple-cascaded systems, however, require additional methane refrigeration compressors, and the savings in feed stream compression is not large enough to provide an overall practical advantage for the cascaded system.
As a result, methane refrigeration has not been widely used in commercial ethylene plants.
Regarding the fractionation steps of the ethylene plant in greater detail, a variety of process equipment sequences are employed. One widely used sequence incorporates a demethanizer column at the head of the fractionation section followed by deethanizer and C.sub.2 splitter columns, depropanizer and C.sub.3 splitter columns, debutanizer column, depentanizer column, and other separation equipment as is required. In another arrangement, a depropanizer is positioned at the head of the fractionation section, so that C.sub.3 's and lighter are separated from C.sub.4 's and heavier initially. This so-called front-end depropanizer scheme generally permits better maintenance of olefin (ethylene) purity specifications, and substantially reduces capital, power, and operating costs over a front end demethanizer arrangement. By separating out the C.sub.4 's and heavier initially, the subsequent separation equipment can operate at extremely low temperatures without the problems arising from freezing of heavy hydrocarbon constituents. Operation at such extremely low temperature levels allows particularly efficient light component separations to be carried out. Similar advantages can be obtained in arrangements using a front-end deethanizer for initial removal of C.sub.3 's and heavier from the hydrocarbon feed gas mixture. In one particularly efficient arrangement for a front-end depropanizer or a front-end deethanizer system, the light components in the depropanizer or deethanizer overhead stream are separated and removed in a forecooling recovery section operating at low temperature, with the remaining ethylene-bearing streams passing to the final separation section. In all of these various arrangements, despite the fact that some savings in refrigeration system compression requirements can be effected by operation at high pressure as compared with low pressure, the ethylene plant has associated therewith extremely large compression system energy requirements.
Accordingly, it is an object of the present invention to provide an improved ethylene production process.
It is another object of the invention to provide an ethylene separation process which is adaptable to a high pressure hydrocarbon feed gas mixture and employs a separation section in which demethanization is carried out at moderate pressure, thereby effecting significant savings in refrigeration compression requirements relative to conventional high pressure ethylene separation processes.
It is another object of the invention to provide an ethylene separation process which substantially reduces the overall operating pressure and hence the total compression energy requirements below the levels associated with prior art high pressure ethylene production processes, while employing only propylene-ethylene refrigeration.
Other objects and advantages of the invention will be apparent from the ensuing disclosure and appended claims.